A semiconductor device using a gallium nitride (GaN) based semiconductor is expected to be applied, from its physical property characteristics, as a device high in breakdown voltage and operable at a high speed, for example, to a server system and so forth.
Among device characteristics of a GaN based semiconductor device such as, for example, a GaN based Schottky barrier diode or a GaN based vertical transistor, the on-resistance and the breakdown voltage are significant and substantially depend upon a drift layer provided in the GaN based semiconductor device.
Incidentally, since the on-resistance and the breakdown voltage have a tradeoff relationship to each other, if the on-resistance decreases, then the breakdown voltage drops. Therefore, in a conventional semiconductor device that uses Si or SiC, a super-junction structure is adopted for the drift layer as depicted in FIG. 28.
The semiconductor device having such a super-junction type drift structure as just described includes a drift layer having a structure that a p-type semiconductor layer and an n-type semiconductor layer are alternately disposed in a horizontal direction (transverse direction) such that a pn junction interface therebetween extends in a vertical direction (longitudinal direction). The drift layer having such a structure as just described is implemented by repetitions of ion implantation and crystal growth or by trench formation, burying growth and polishing. In the semiconductor device including such a super-junction type drift structure as just described, the on-resistance can be reduced by increasing the donor concentration of the n-type semiconductor layer. However, when the semiconductor device is in an off state, a depletion layer extending along the pn junction interface spreads in a horizontal direction, and therefore, the breakdown voltage can be raised.